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Mass movements in the Rio Grande Valley (Quebrada de , Northwestern ): a methodological approach to reduce the risk

G. Marcato1, A. Pasuto1, and F. R. Rivelli2 1CNR-IRPI – National Research Council of Italy, Research Institute for Hydrological and Geological Hazard Prevention, C. so Stati Uniti 4, 35127 Padova, Italy 2Departamento de Ciencias Naturales, Universidad Nacional de Salta, Casilla De Correo 529, 4400 Salta, Argentina Received: 9 June 2009 – Revised: 24 August 2009 – Accepted: 25 August 2009 – Published: 13 October 2009

Abstract. Slope processes such as slides and debris flows, will bridge the gap between the necessity of preservation and are among the main events that induce effects on the Rio the request of safety keeping of the recommendation by UN- Grande sediment transport capacity. The slides mainly af- ESCO. fect the slope of the Rio Grande river basin while debris and mud flows phenomena take place in the tributary valleys. In the past decades several mass movements occurred causing victims and great damages to roads and villages and there- 1 Introduction fore hazard assessment and risk mitigation is of paramount The Rio Grande flows in a narrow mountain valley located in importance for a correct development of the area. This is the Province of Jujuy in northwest Argentina, some 1500 km also an urgent need since the was north-west of . This valley, named Quebrada recently included in the UNESCO World Cultural Heritage. de Humahuaca (QdH), follows the line of a major cultural The growing tourism business may lead to an uncontrolled route, the Camino Inca, from the Rio Grande springs in the urbanization of the valley with the consequent enlargement cold high desert plateau of the High Andean lands, to its con- of threatened areas. fluence with the Rio Leone some 150 km to the south. It has In this framework mitigation measures have to take into provided a natural route for people to travel from the time of account not only technical aspects related to the physical be- hunters and gatherers right to the present day. For this reason haviour of the moving masses but also environmental and it has been included in the UNESCO Word Heritage since sociological factors that could influence the effectiveness of 2003. the countermeasures. Mitigation of landslide effects is indeed rather complex There are almost 20 hamlets along the valley, but the because of the large extension of the territory and the par- population is dramatically shrinking due to the high fre- ticular geological and geomorphological setting. Moreover quency of mass movements that cause relevant damages to the necessity to maintain the natural condition of the area as the Ruta Panamericana Nacional n. 9, connecting Argentina prescribed by UNESCO, make this task even more difficult. with and , and the Ferrocarril General Belgrano. Nowadays no in-depth study of the entire area exists, This strategic railway was completely abandoned in 1992 therefore an integrated and multidisciplinary investigation due to high maintenance costs. Moreover, these mass move- plan is going to be set up including geological and geo- ments cause serious damages also to the farming activities morphological investigations as well as archaeological and along the valley floor actually reducing the land available to historical surveys. The better understanding of geomorpho- be cultivated and therefore the possibility of sustenance of logical evolution processes of the Quebrada de Humahuaca the population. Landslides and debris flows are thus the main hazardous phenomena affecting the slopes and inducing serious con- Correspondence to: G. Marcato sequences on the erosion and sedimentation activity of the ([email protected]) Rio Grande (Cencetti et al., 2001). A great amount of loose

Published by Copernicus Publications on behalf of the European Geosciences Union. 60 G. Marcato et al.: Mass movements in the Rio Grande Valley (Quebrada de Humahuaca, Northwestern Argentina) debris is available due to the geological and lithotechnical 175 mm/yr, respectively). The amount of rainfall increases characteristics of the outcropping geologic units and this in- northward, in the Puna Jujena region, with 282 mm/yr in duces a dangerous situation for the infrastructures and the up to 335 mm/yr in , near to the Bol- population. vian border. Nowadays, an in-depth study of the entire area is still not The mean annual temperature in the central part of the val- available, therefore an integrated and multidisciplinary inves- ley, near Humahuaca, for the period 1961–1970, is of the tigation plan is going to be set up. order of 12◦C. Southward, in Hornillos, it reaches 14◦C, The research activity should include geological and geo- whereas in the northern areas (La Quiaca), the average value morphological investigations as well as archaeological and lowers to 9.2◦C, because of the higher altitudes. The thermal historical surveys. Moreover, social and economical aspects regime is anyway characterized by significant variations on a have to be considered in order to assess the impact of mitiga- daily basis, with differences that can reach 40◦C. tion measures on the urban and anthropic environment. This thermal regime plays the major role in the production of debris at the foot of slopes, that is made available and mobilized during debris flow and flood events. 2 Physical setting The Rio Grande, while crossing the QdH, receives the wa- ters of right hand tributaries, such as the Reyes, Leon Pur- The Quebrada de Humahuaca is included in the ridge-and- mamarca and Yacoraite streams. Most of these streams orig- valley system with a N-S direction, characterizing the eastern inate from springs located at altitudes in the order of 4000 m, Cordillera Andina. Its outlet is located close to San Juan de where permanent glaciers guarantee an outflow during the Jujuy. The valley represents a strategic route connecting Ar- whole year. gentina to Chile and Bolivia (Fig. 1). It is crossed by the Rio Grande, which is characterized by a variable discharge due to extreme climatic variability, either in space and in time. The 3 Geological and geomorphological outlines historic time series of river discharge is short and discontin- uous (from 1940 to 1945 and 1951 to 1954). The station The geological and structural features of the Quebrada de of San Juan is located some 180 km downstream from the Humahuaca are relatively simple: from the valley bottom up springs, and the basin extends for 7650 km2. It has an av- to the western ridge, Precambrian and Cambrian units pre- erage width of 300 m and a slope of 1.30%. The mean daily vail. The first belongs to the Puncoviscana Formation, a low discharge varied from 16.4 and 24.7 m3 s. The maximum dis- grade metamorphic unit with greywacke; it is characterized charge was 358 m3 s and the minimum was 3 m3 s. by a one thousand meters thickness an intense and diffuse The elevation of the valley bottom ranges from fracturing. The Cambrian rocks, belonging to the Meson 3440 m a.s.l. in the vicinity of the village of Iturbe to 1350 m group, which is itself divided into three subunits, constitute in correspondence to its confluence to Rio Reyes; the total the principal core of the ridge (sierra). These are mainly length is about 144 km and its width ranges from 100 m up multicolored orthoquarzites and quarz-rich arenites (Ramos to 3 km close to the village of . The valley is bounded et al., 1967; Turner, 1970; Amegual and Zanettini, 1974). by the Sierra Zenta-Tilcara eastward, that reaches the eleva- Over these units, an extensive Quaternary cover is found, tion of 5100 m and the Sierra Agular and Chani, westward which was generated by gravity processes acting in the up- exceeding 6200 m. Therefore, the differences in elevation per part of the slopes together with debris flows and alluvial are very pronounced and strongly influence the distribution phenomena in the valley bottom. Mass wasting phenomena of precipitations. are also conditioned by the notable tectonic uplift of the area From the climatic point of view, the valley is characterized (Chayle et al., 1987). by a sub-tropical regime in its southernmost part, with an The general morphology is related to the presence of a ho- average yearly rainfall of more than 400 mm, whereas the moclinal, with N-S direction dipping eastward. Regional N- central part, between the hamlets of Tilcara and Humahuaca, S faults have produced the uplift of large areas and deter- is dryer and can be considered a semi-arid environment. This mined the development and incision of N-S valleys, such as is also due to the great differences in elevation between the the Quebrada de Humahuaca (Turner, 1970; Mon and Salfity, valley bottom and the surrounding peaks. 1995). The synthesis of the monthly rainfall in the period 1934– The morphology of this area is strictly controlled by the 1990 is given in Fig. 1, where the total lack of precipitation lithological features of the rock masses and by ancient and during winter and the summer peak can be observed. active tectonics. The landscape is characterized by a signifi- Rainfall data recorded in the period 1934–1990 shows cant asymmetry of the valley flanks which in turn conditions an average of 391 mm/yr in Volcan, with a maximum of the type and activity of the processes and the landforms. 719 mm in 1963, and a minimum of 123 mm during 1940. On the eastern flank two terraces formed by Rio Grande With reference to the same period, average values lower than can be found, together with huge incised alluvial fans gener- 200 mm have been recorded in Tilcara-Humahuaca (136 and ated by the different tributaries originating from the eastern

Adv. Geosci., 22, 59–65, 2009 www.adv-geosci.net/22/59/2009/ G. Marcato et al.: Mass movements in the Rio Grande Valley (Quebrada de Humahuaca, Northwestern Argentina) 61

Fig. 1. Location map of Northwest Argentina. The Quebrada de Humahuaca stretches across the Tropic of Capricorn. Monthly rainfall in the period 1934–1990 in the Quebrada de Humauaca and Puna Jujena Regions. Data are derived from the National Meteorological Survey and from the Ferrocaril General Belgrano. Main roads, including Ruta Nacional n. 9 Panamericana, are indicated on the map. www.adv-geosci.net/22/59/2009/ Adv. Geosci., 22, 59–65, 2009 62 G. Marcato et al.: Mass movements in the Rio Grande Valley (Quebrada de Humahuaca, Northwestern Argentina)

Fig. 2. Sketch of the main geomorphological processes and landforms active in the Quebrada de Humahuaca at present. cordillera. The present-day geomorphological evolution of can be stated that no alluvial phenomena involving the entire the Quebrada de Humahuaca is largely controlled by the valley occurred in the last centuries. This is mainly due to mobilization of the thick deposits generated by the physi- the particular distribution of the rainfall that is strictly con- cal weathering of the rock masses. The presence of a large trolled by the relief and difference in elevation. It is rather amount of debris facilitates debris flow and hyperconcen- common to register heavy rainfall on the east slope and dry trated flow phenomena, that lead to the aggradations of the conditions on the opposite. It follows that the occurrence in Rio Grande watercourse. The Ruta Nacional n. 9 is involved time and space of debris flows and other mass movements is in some small landsliding phenomena (Fig. 2). quite random and unpredictable. Nevertheless, several catastrophic events, causing eco- nomic losses and also victims, occurred in the last decades. 4 Main hazard factors and risk implications Noteworthy is the debris flow that occurred in 1983 in the As pointed out by the description of the geomorphological Rio Huasamayo river basin. It involved the village of Tilcara evolution, the main processes in the Quebrada de Humahuaca overflooding the cultivated fields and the camping area where are related to mass transport, that originates from the right two tourists lost their life. The estimated volume was of hand side tributaries, and to erosional and depositional pro- the order of some million of cubic meters and the deposits cesses related to the Rio Grande fluvial system. dammed the Rio Grande valley, causing the formation of a Under the present morphoclimatic context, these predis- lake. posing factors lead to extensive alluvial phenomena at the One year later a similar event occurred in the Rio Pur- outflow of the tributary valleys and in the main valley floor, mamarca basin; approximately 2.5 million cubic meters of where large alluvial fans have occasionally determined the debris dammed the Rio Grande causing one victim, serious partial or total damming of the river, as it can be observed damages to infrastructure such as road and bridges and the in the area of El Volcan. In the recent past, some of these burial of several km of railway track. A deposition area of fans have been exploited as preferential areas for hamlets and about 200 m×3000 m, with an average thickness of 4–5 m, houses, with subsequent risk conditions posed to population. was mapped after the event. Apart from the risk posed to the inhabited areas in the vicin- Concerning debris flow phenomena, the most evident ity of streams, a further consequence of alluvial phenomena landform within the Rio Grande Valley is the huge fan of is the progressive reduction of agricultural lands. Arroyo del Medio, located between Leon and Volcan, at al- It is interesting to analyze the sedimentary sequences out- titudes ranging from 1600 and 2000 m. Its area is about cropping along the Rio Grande valley since they highlight an 9 km2, creating a morphological step of more than 300 m almost systematic recurrence of mass movements. Unfortu- (Harrington, 1946). The Arroyo del Medio rises at an alti- nately no radiometric datings are available at present so that tude of 4300 m and, after more than 10 km, flows into the it is not possible to define frequency and return time, but it Rio Grande at an altitude of 2000 m (Gonzales et al., 1987).

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The debris flow source area is a large and semicircular deep 5 Integrated methodological approach to mitigate incision located at the foot of Chani Chico (6200 m), and is the risk characterized by large differences in heights (between 4300 and 2900 m), and steep slope (about 40◦). The flows initiated The Quebrada de Humahuaca is characterized by a huge vol- in this area involve the thick Pleistocenic deposits (Spalletti, ume of highly movable debris, causing different typologies 1968). The channel is cut into older deposits and it is up to of mass movements namely debris flows, slides and falls. 50 m deep in the upper part, while downslope the depth is in In particular, debris flow phenomena have great impact on the order of 10 m. infrastructures and agricultural activities thus reducing the The accumulation zone is 8 km wide, with an average chances of future development of the area. The valley has slope of 5◦. The granulometry of the deposits is quite varying provided a natural route for people to travel from the time of with respect to different phenomena (Monteverde and Kittl, hunters and gatherers right to the present day. For this reason 1946). it has been included in the UNESCO Word Heritage since The fan dammed the Rio Grande several times, as docu- 2003 with to the following requests: “to submit to the World mented by the lacustrine deposits located upstream close to Heritage Centre the final management plan and encourage the village of Volcan. These clayey and silty deposits are the State Party to carry out an environmental assessment of presently cut by the river forming a canyon up to 30 m deep the proposed flood defence project in order to gauge its im- (Cencetti et al., 2004). pact on the outstanding universal value of the Valley”. In this area, a huge debris flow occurred during the sum- In this framework, a mitigation strategy aimed at avoiding mer season of 1945 caused the blockage of the Rio Grande potential damages should be implemented in order to pre- and the formation of a lake which flooded the village dam- serve the natural condition of such a fragile environment, aging all the houses and the railway station. After this event even if it seems quite complex because of the extent of the it was decided to blast a new epigenetic river bed into the area to investigate and because of the close relationship be- bedrock. Unfortunately the effects of the new hydraulic con- tween natural and socio-economic environment. ditions were not calculated, and thus evident processes of ret- Given this situation two different needs have to be guaran- rogressive erosion, that still continue today, were triggered. teed: This is really unusual, as these are the only active erosion – to preserve the landscape as it is without any human in- processes in the Quebrada de Humahuaca where mainly de- tervention; positional processes take place. In 1984 and 1985 several debris flow phenomena occurred – to protect the populations and the human activities in in the Arroyo del Medio catchment as well. In particular, order to provide them future chances of development. the event of February 1985 caused the complete destruction of a railway bridge and the interruption of the N. R. n. 9. An in-depth study of the entire area is not available at More than 4 m thick deposits covered a large area with up to present, therefore an integrated and multidisciplinary investi- 10 cubic meters blocks. A volume of about 2.4 million cubic gation plan is going to be set up. The research activity should meters was estimated. Following these events the National include geological and geomorphological investigations as Road was shifted to the fan apex and the Railway “General well as archaeological and historical surveys. Moreover, so- Belgrano” was completely abandoned some years later. cial and economical aspects have to be considered in order These phenomena, together with hyperconcentrated flow to assess the impact of defence measures on the urban and processes, have deposited a huge volume of sediments in the anthropic environment. Rio Grande valley, hindering the normal flow of the river and It is not convenient, as it happens nowadays, to design posing subsequent risky condition for the villages located countermeasures for single phenomena without considering along the river. all the aspects of the whole geomorphological evolution of In this framework, the N. R. n. 9 and the villages have to the valley. A comprehensive “plan of actions” has to take be considered now the main vulnerable elements. The road into account the possibility to reduce the deposition of debris is threatened either by landslide and debris flow phenomena flow materials in the Rio Grande River bed, trapping it along or by river erosion since it runs close to the river bed. As the tributaries. In this way the magnitude of the depositional already mentioned, it was rebuilt several times due to over- processes as well as the disequilibrium in the hydrodynamics flooding and erosion processes. But the new route is now of the Rio Grande could be reduced. affected by landslides, mainly caused by the undercutting of The slope failures pose serious problems mainly along the slopes. Generally these landslides are usual shallow and the N. R. n. 9. More than 100 km of road is affected by rota- involved volume is of the order of thousands of cubic me- tional slide and rock fall. This was not considered when the ters; nevertheless they cause serious problems for the traffic new route of the road was designed, shifting it upslope to and for the safety of population with risk conditions that are avoid flooding phenomena but posing it to landslide hazard. anyway not acceptable, because of the strategic importance An in-depth evaluation on the more suitable typology of re- of the road (Fig. 3). taining structures has to be done since those located along www.adv-geosci.net/22/59/2009/ Adv. Geosci., 22, 59–65, 2009 64 G. Marcato et al.: Mass movements in the Rio Grande Valley (Quebrada de Humahuaca, Northwestern Argentina)

Fig. 3. Quebrada de Humahuaca watershed. The main tributaries and villages along the valley are indicated. (a) A detail of Rio Huasamayo alluvial fan, where Tilcara village is located. The village is permanently threatened by the debris flow phenomena. (b) Quebrada de Trancas, close to Maimara` village. At the confluence with Rio Grande, the alluvial fan generation, due to the recurrent debris flow phenomena, causes the progressive deviation of the Rio Grande riverbed, which, in turn, tends to erode the right river bank. It was necessary to set up a series of gabions in order to protect the N. R. n. 9 Panamericana. (c) Rio Grande riverbed, between Timbaya and Volcan. Due to the alluvial aggradation, the Ferrocaril General Belgrano and the old path of Ruta Nacional were abandoned in 1992. The new road has been rebuilt upslope. (d) n. 9 new route is under a continuous maintenance, due to the frequent debris slides triggering by undercutting processes.

Adv. Geosci., 22, 59–65, 2009 www.adv-geosci.net/22/59/2009/ G. Marcato et al.: Mass movements in the Rio Grande Valley (Quebrada de Humahuaca, Northwestern Argentina) 65 the road show great differences either in terms of efficiency Cencetti, C., Rivelli, F. R., Tacconi, P., and Viglione, F.: La Que- or environmental impact. brada de Humahuaca (Bacino del Rio Grande de Jujuy, Ar- For all these reasons a multidisciplinary team including gentina nord-occidentale): caratteristiche di un bacino di ambi- geologists, geomorphologists, sociologists, archaeologists, ente andino, L’Universo, 4, 496–514, 2001. landscape architects, human geographers has been appointed Chayle, W. and Aguero, P.: Caracteristicas de la remocion en masa for providing a comprehensive study of all the aspects re- en la Cuenca del Rio Grande (quebrada de Humahuaca, Jujuy), Revista del Instituto de Geologia y Mineria, San Salvador de Ju- lated to the interaction between environment and human set- juy, Universidad Nacional de Jujuy, 7, 24–31, 1987. tlements. The main aims are to better understand the geo- Gonzales Diaz, E. and Fauque, L. E.: Proveniencia del material morphologic processes and the socio-economic development componental del torrente de barro de “El Volcan”, Quebrada in order to design a sustainable mitigation plan. Special at- de Humahuaca (Jujuy), Repubblica Argentina, Proc. of Decimo tention will be paid to develop tools and methods to increase Congreso Geologico Argentino, San Miguel de Tucuman, III, the public awareness on landslide hazard and to built a re- 309–312, 1987. silient community in such a disaster-prone area. This multi- Harrington, H. J.: Las corrientes de barro (mud flow) del “El Vol- disciplinary approach to the slope instability problems of the can” Quebrada de Humahuaca, Jujuy, Asoc. Geol. Arg. Rev., Quebrada de Humahuaca should bridge the gap between the 1(2), 149–165, 1946. necessity of preservation and the request of safety, keeping Mon, R. and Salfity, J. A.: Tectonic evolution of the of North- in mind the recommendation of UNESCO. ern Argentina, in: Petroleum basins of South America, edited by: Tankard, A. J., Suarez, R. S., and Welsink, H. J., American As- Acknowledgements. The work presented in this paper was carried sociation of petroleum Geologist Memoir, 62, 269–283, 1995. out with funding from National Research Council of Italy, Interna- Monteverde, A. and Kittl, E.: Avalancha “El Volcan”, Trabajo, 46, tional short mobility Program for scientist/researchers from Italian 891–956, 1946. and Foreign Institutions, grant number 0030469/2006. Ramos, V. A., Turic, M. A., and Zuzek, A. B.: Geologia de las Que- bradas de Huichara-Pocoya, y Tumbaya Grande en Edited by: P. Fabian la Margen Derecha de la Quebrada de Humahuaca (provincia Reviewed by: two anonymous referees de Jujuy), revista de la Asociacion Geologica Argentina, 22(3), 209–221, 1967. Spalletti, L. A.: Sedimentologia de los cenoglomerados de Volcan. References Provincia de Jujuy. Revista del Museo de La Plata, Universidad Nacional de La Plata, 66(VIII), 137–152. 1968. Amegual, R. and Zanettini, J. C.: Geologia de la quebrada de Turner, J. C. M.: The Andes of Northwestern Argentina, Geol. Humahuaca entre Uquia y Purmamarca (provincia de Jujuy), Re- Rundsch., 59(3), 1028–1063, 1970. vista de la Asociacion Argentina, 29(I), 30–40, 1974. Cencetti, C., Rivelli, F. R., and Tacconi, P.: Le colate detritiche nell’Arroyo del Medio (Provincia di Jujuy, Argentina nord- occidentale), Proc. of “INTERPRAEVENT 2004” (Riva del Garda, Trento, 24–28 maggio 2004), 1(III), 45–56, 2004.

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